1. Field of the Invention
This invention relates to optical fibers and, more particularly, to large-mode-area (LMA) optical fibers with reduced bend distortion.
2. Discussion of the Related Art
The ever increasing demand for higher optical power output from fiber amplifiers and lasers has stimulated extensive research and development, pushing the limits of fiber design. Better fibers are key to improving amplifier performance, and LMA designs in particular increase power allowed in the face of limitations imposed by nonlinear effects [e.g., A. Galvanauskas, IEEE J. Sel. Top. Quantum Electron., Vol. 7, pp. 504-517 (2001) and C. C. Renaud et al., IEEE J. Quantum Electron., Vol. 37, pp. 199-206 (2001), both of which are incorporated herein by reference]. To move beyond conventional performance, researchers have refined fabrication limits, tested traditional assumptions, and explored various design approaches [e.g., S. Ramachandran et al., Opt. Lett., Vol. 31, pp. 1797-1799 (Jun. 15, 2006); P. Wang, et al., Opt. Lett., Vol. 31, pp. 226-228 (Jan. 15, 2006); W. S. Wong, et al., Opt. Lett., Vol. 30, pp. 2855-2857 (2005); L. Zenteno, et al., Opt. Express, Vol. 13, pp. 8921-8926 (2005); C. J. S. de Matos, Opt. Express, Vol. 11, pp. 2832-2837 (2003), and K. Furusawa, et al., Opt. Express, Vol. 9, pp. 714-720 (2001), all of which are incorporated herein by reference].
One of the assumptions implicit in conventional fiber amplifier/laser designs is that a bent gain fiber experiences macrobending loss (for bends with relatively constant curvature along the fiber length) and mode coupling (for microbends or bend transitions with more variable curvature along the fiber length) but sees no other impact important to amplifier performance. Distortion of the optical mode profile in response to a fiber bend is known [e.g., J. C. Baggett, et al., Opt. Commun., Vol. 227, pp. 317-335 (2003), which is incorporated herein by reference] but generally has been neglected in amplifier fiber design and characterization. This assumption eventually breaks down as core size increases, and the current aggressive push to larger mode area has already put amplifier designs in a regime where bend distortion must be considered.
In fact, simply bending a fiber onto a spool of any reasonable package size produces large bend distortion for conventional fibers with core diameter ˜50 μm or greater. Because this distortion reduces the effective mode area, it directly impacts amplifier performance and partially defeats the purpose of using a large area core to begin with. The realization that bend distortion is of growing importance highlights the difficulty of making extremely large mode area fibers practical, and qualitatively it changes the design strategies needed to get good performance. Naturally, one solution to the bend distortion problem is to keep the fiber straight, but this approach may be impractical for many applications, especially when fiber lengths are one meter or more.
Thus, a need remains in the art for a method of making a LMA fiber that effectively reduces bend distortion.
To the extent that the prior art workers have considered bend distortion they have generally done so in a limited way, ignoring how it adversely impacts the interaction between the gain region and the signal light to be amplified.
Thus, there is also a need for a gain-producing LMA fiber that not only effectively reduces bend distortion but also preserves the interaction between the signal light and the gain region.